Some communication links vary their data transmission rate, such as in response to varying channel conditions. Various methods and systems for operating variable-rate communication links are known in the art. For example, U.S. Patent Application Publication 2005/0075078, whose disclosure is incorporated herein by reference, describes a point-to-point packet microwave radio link. In order to improve the efficiency on the radio link, packets are classified before transmission based on at least one Quality-of-Service (QoS) parameter assigned to each packet. The signals are modulated with a real-time adaptive modulation. The modulation is adapted based on the current traffic amount, on signal quality measurements indicative of the propagation conditions on the radio link, and on the classification of packets comprised in the signals.
Many communication systems and services use continuous synchronous connections, also referred to as synchronous tributaries. Some synchronous connections carry data in accordance with a certain Time-Division-Multiplexing (TDM) protocol, in which multiple data streams are multiplexed together. In some cases, the data streams are multiplexed in a hierarchical structure.
Synchronous connections may comprise, for example, T1 connections or E1 connections. Higher rate synchronous connections may comprise multiple T1 or E1 carriers, such as T2, T3 and E2, E3 links. (T1, T2, . . . links are also referred to as DS1, DS2, . . . ) The DS1, DS2 and DS3 links are specified in standard T1.102-1993 of the American National Standards Institute (ANSI®) entitled “American National Standard for Telecommunications—Digital Hierarchy—Electrical Interfaces,” December, 1993, which is incorporated herein by reference.
In some applications, multiple synchronous digital connections are multiplexed to produce high data rate synchronous connections, such as in the Plesiochronous Digital Hierarchy (PDH), Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) architectures. The PDH architecture is described, for example, in recommendation G.703 of the International Telecommunication Union (ITU) entitled “General Aspects of Digital Transmission Systems—Terminal Equipments—Physical/Electrical Characteristics of Hierarchical Digital Interfaces,” Geneva, Switzerland, 1991, and in recommendation G.704 of the ITU Telecommunication Standardization Sector (ITU-T), entitled “Series G: Transmission Systems and Media, Digital Systems and Networks, Digital Transmission Systems—Terminal Equipments—General, Synchronous Frame Structures used at 1544, 6312, 2048, 8448 and 44736 Kbit/s Hierarchical Levels,” October, 1998, which are incorporated herein by reference.
The SDH architecture is described, for example, in ITU-T recommendation G.783, entitled “General Aspects of Digital Transmission Systems; Terminal Equipments—Characteristics of Synchronous Digital Hierarchy (SDH) Equipment Functional Blocks,” January, 1994, and in ITU-T recommendation G.707/Y.1322 entitled “Series G: Transmission Systems and Media, Digital Systems and Networks, Digital Terminal Equipments—General; Series Y: Global Information Infrastructure and Internet Protocol Aspects, Internet Protocol Aspects—Transport Network Node Interface for the Synchronous Digital Hierarchy (SDH),” which are incorporated herein by reference.
SONET is described, for example, in standard GR-253-CORE by Telcordia™ Technologies (Piscataway, N.J.), entitled “Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria,” Issue 3, September, 2000, chapters 1-4, and in a standard published by the Alliance for Telecommunications Industry Solutions (ATIS) entitled “Synchronous Optical Network (SONET)—Basic Description including Multiplex Structure, Rates and Format,” 2002, which are incorporated herein by reference.